U.S. patent number 5,903,198 [Application Number 08/902,702] was granted by the patent office on 1999-05-11 for planar gyrator.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Jerald A. Weiss.
United States Patent |
5,903,198 |
Weiss |
May 11, 1999 |
Planar gyrator
Abstract
In a planar gyrator, parallel transmission lines are positioned
proximal to a magnetized gyrotropic substrate. Input and output
transducers couple the ends of the transmission lines to
corresponding input and output ports. The input and output
transducers are configured to excite first and second partial wave
fields on the transmission lines of similar or different phases
respectively. The wave fields, in turn, interact gyromagnetically
with the substrate, such that the resultant difference in phase
change for a first wave propagating from the first to the second
port and a second wave propagating from the second to the first
port is an odd-integer multiple of 180 degrees. Alternatively, if
the magnetization of the substrate is reversed, the phase of a wave
propagating from the first to the second port is changed by 180
degrees. The planar gyrator is amenable to application in
miniaturized planar microwave devices, for example as a
magnetically-controlled phaser or switch, or as a component in a
circulator or isolator implemented in planar microwave
technology.
Inventors: |
Weiss; Jerald A. (Wayland,
MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
25416263 |
Appl.
No.: |
08/902,702 |
Filed: |
July 30, 1997 |
Current U.S.
Class: |
333/24.1;
333/99R |
Current CPC
Class: |
H01P
1/32 (20130101) |
Current International
Class: |
H01P
1/32 (20060101); H01P 001/32 () |
Field of
Search: |
;333/1.1,24.1,24.2,102,26 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Tellegen, B.D.H., (Apr., 1948) "The Gyrator, a New Electric Network
Element," Phillips Research Reports 3: 81-101. .
Polder, D., (1949) "On the Theory of Ferromagnetic Resonance," Phil
Mag, 40: 99. .
Hogan, C. L. (Jan., 1952) "The Ferromagnetic Faraday Effect at
Microwave Frequencies and its Applications," The Bell System
Technical Journal 31(1). .
Hogan, C. L. (Jan., 1953) "Ferromagnetic Faraday Effect at
Microwave Frequencies and its Applications," Reviews of Modern
Physics 25(1): 253-263. .
Carlin, H. J. (May, 1955) "On the Physical Realizability of Linear
Non-Reciprocal Networks," Proceedings of the IRE: 608-616. .
Weiss, J.A. (May, 1960) "Tetrahedral Junction," Journal of Applied
Physics Supplement 31(5): 168S-169S. .
Mesa, F. (1991) "Rigurous Analysis of Non-Reciprocal Slow-Wave
Planar Transmission Lines," 21st European Microwave Conference 1:
229-234. .
Fitzgerald, W. D. (1992) "A 35-GHz Beam Waveguide System for the
Millimeter-Wave Radar,"The Lincoln Laboratory Journal 5(2):
245-271. .
Lax, B. (Dec., 1993) "Quasi-Optical Ferrite Reflection Circulator,"
IEEE Transactions on Microwave Theory and Techniques 41(12):
2190-2197. .
Weiss, J. A. et al. (Jan. 10, 1994) "Quasi-optical Reflection
Circulator: Progress in Theory and Millimeter-Wave Experiments,"
International Society for Optical Engineering, 2211: 416-427. .
Bloch, F., et al., "The Nuclear Induction Experiment" Phys. Rev.
vol. 70, Nos. 7 & 8, (1946), pp. 474-485. .
Kittel, C., "On the Theory of Ferromagnetic Resonance Absorption,"
Phys. Rev. vol. 73, No. 2, (1948) pp. 155-161. .
Kittel, C., "On the Gyromagnetic Ratio and Spectroscopic Splitting
Factor of Ferromagnetic Susbstances," Phys. Rev. vol. 76, No. 6,
(1949) pp. 743-748. .
Ramsey, N.F. "Paramagnetic and Ferromagnetic Resonance," Nuclear
Moments, John Wiley & Sons, Inc., (1953), pp. 129-136. .
Lax, B., et al., "Farraday-Rotation Devices: Physical Description,"
Microwave Ferrites and Ferrimagnetics; McGraw-Hill, Sec. 12-1,
(1962) pp. 544-549. .
Roome, G.T., et al., "Thin Ferrite Devices for Microwave Integrated
Circuits," IEEE Transactions on Microwave Theory and Techniques,
vol. MIT-16, No. 7, (1968) pp. 411-420. .
Rosenbaum, F.J., "Integrated Ferrimagnetic Devices," Advances in
Microwaves,vol. 8, Academic Press (1968) pp. 203-294..
|
Primary Examiner: Gensler; Paul
Attorney, Agent or Firm: Lappin & Kusmer LLP
Government Interests
GOVERNMENT SUPPORT
The United States government has rights in this invention pursuant
to Contract Number F19628-95-C-0002, awarded by the United States
Air Force.
Claims
I claim:
1. An electromagnetic device comprising:
first and second substantially parallel conductors for supporting
first and second elliptically polarized normal-mode wave fields
propagating in substantially opposite chirality;
a first transducer coupling a first port to the first and second
conductors such that an electromagnetic wave incident at the first
port excites first and second normal-mode partial wave fields on
the conductors with substantially equal amplitude;
a gyrotropic medium sufficiently proximal to the conductors to
introduce gyromagnetic interaction with the wave fields, said
gyromagnetic interaction causing unequal phase change of the first
and second partial wave fields during propagation; and
a second transducer coupling the first and second conductors to a
second port, such that the unequal phases of the partial wave
fields arriving at the second transducer are compensated so as to
constructively interfere at the second port, the resultant
difference in phase change on transmission for a first wave
propagating from the first to the second port and a second wave
propagating from the second to the first port being substantially
an odd-integer multiple of 180 degrees.
2. The device of claim 1 wherein the first transducer couples the
first port to the conductors such that said electromagnetic wave
incident at said first port excites said first and second partial
wave fields on the conductors with a predetermined first difference
in phase.
3. The device of claim 1 wherein the first transducer couples the
first port to the conductors such that said electromagnetic wave
incident at said first port excites said first and second partial
wave fields on the conductors with substantially equal phase.
4. The device of claim 1 wherein the second transducer couples the
second port to the conductors such that a second electromagnetic
wave incident at said second port excites said first and second
partial wave fields on the conductors with a predetermined second
difference in phase.
5. The device of claim 1 wherein the first and second parallel
conductors are superconductors operating in a superconducting
state.
6. The device of claim 1 wherein the conductors are planar
conductors.
7. The device of claim 6 wherein the conductors are
photolithographically deposited on a surface of the gyrotropic
medium.
8. The device of claim 1 wherein the conductors are shaped to
reduce conduction loss.
9. The device of claim 1 wherein the gyrotropic medium includes a
state of magnetization which is reversible in direction to cause
the unequal phase changes of the first and second partial wave
fields to be interchanged, resulting in a reversal of direction of
gyrator action.
10. The device of claim 1 wherein the gyrotropic medium includes a
state of magnetization which is variable between forward and
reverse saturation levels.
11. The device of claim 1 wherein the second transducer includes a
balun structure for compensating for unequal phase changes in the
first and second partial wave fields.
12. A gyrator comprising:
a gyrotropic substrate;
first and second substantially parallel conductors sufficiently
proximal to said substrate such that wave fields traversing the
conductors interact gyromagnetically with the substrate in a zone
of gyromagnetic interaction between said conductors;
a first transducer coupling a first port to first ends of said
first and second conductors such that an electromagnetic wave
incident at said first port excites first and second partial wave
fields propagating in substantially opposite chirality on said
conductors with substantially equal amplitude; said first and
second partial wave fields undergoing unequal phase changes during
propagation through said zone; and
a second transducer coupling a second port to second ends of said
first and second conductors, such that said first and second
partial wave fields of unequal phases reinforce at said second
port.
13. The gyrator of claim 12 wherein the resultant difference in
phase change for a first electromagnetic wave propagating from said
first to said second port and a second wave propagating from said
second to said first port is substantially an odd-integer multiple
of 180 degrees.
14. The gyrator of claim 12 wherein said first and second partial
wave fields are elliptically polarized.
15. The gyrator of claim 12 wherein said conductors are
substantially planar.
16. The gyrator of claim 12 wherein said gyrotropic substrate
includes a state of magnetization.
17. The gyrator of claim 16 wherein said magnetization is
reversible in direction to cause the unequal phase changes of the
first and second normal modes to be interchanged, resulting in a
reversal of direction of gyrator action.
18. The gyrator of claim 16 wherein said magnetization is variable
between forward and reverse saturation levels.
19. The gyrator of claim 12 wherein said second transducer includes
a balun structure for compensating for unequal phase changes in the
first and second partial wave fields.
20. An electromagnetic device having first and second ports wherein
a first electromagnetic wave propagating from the first to the
second port and a second electromagnetic wave propagating from the
second to the first port undergo respective phase changes which are
different by substantially an odd-integer multiple of 180 degrees,
said device comprising:
a gyrotropic substrate;
first and second substantially parallel conductors supporting first
and second partial wave fields propagating in opposite chirality,
said conductors sufficiently proximal to said substrate such that
said partial wave fields traversing said conductors interact
gyromagnetically therewith;
a first transducer coupling said first port to first ends of said
first and second conductors; and
a second transducer coupling said second port to second ends of
said first and second conductors.
21. The device of claim 20 wherein said first transducer excites
said first and second partial wave fields on said conductors with
substantially equal phase upon said first electromagnetic wave
being incident at said first port.
22. The device of claim 20 wherein said second transducer excites
said first and second partial wave fields on the conductors with
substantially opposite phases upon said second electromagnetic wave
being incident at said second port.
23. The device of claim 20 wherein said first transducer produces
constructive interference of said first and second partial wave
fields of substantially equal phase at said first port.
24. The device of claim 20 wherein said second transducer produces
constructive interference of said first and second partial wave
fields of substantially opposite phase at said second port.
25. A method for forming an electromagnetic device having first and
second ports such that a first electromagnetic wave propagating
from the first to the second port and a second electromagnetic wave
propagating from the second to the first port undergo respective
phase changes which are different by substantially an odd-integer
multiple of 180 degrees comprising the steps of:
disposing first and second substantially parallel conductors
supporting first and second partial wave fields of substanially
equal amplitude propagating in opposite chirality in sufficient
proximity with a gyrotropic substrate such that said partial wave
fields traversing said conductors interact gyromagnetically
therewith;
coupling a first transducer between said first port and first ends
of said first and second conductors; and
coupling a second transducer between said second port and second
ends of said first and second conductors.
26. An electromagnetic device comprising:
first and second substantially parallel conductors for supporting
first and second elliptically polarized normal-mode wave fields
propagating in substantially opposite chirality;
a first transducer coupling a first port to the first and second
conductors such that an electromagnetic wave incident at the first
port excites first and second normal-mode partial wave fields on
the conductors;
a gyrotropic medium sufficiently proximal to the conductors to
introduce gyromagnetic interaction with the wave fields, said
gyromagnetic interaction causing unequal phase change of the first
and second partial wave fields during propagation; and
a second transducer coupling the first and second conductors to a
second port, said second transducer including a balun structure
such that the unequal phases of the partial wave fields arriving at
the second transducer are compensated so as to constructively
interfere at the second port, the resultant difference in phase
change on transmission for a first wave propagating from the first
to the second port and a second wave propagating from the second to
the first port being substantially an odd-integer multiple of 180
degrees.
27. An electromagnetic device comprising:
first and second substantially parallel conductors for supporting
first and second elliptically polarized normal-mode wave fields
propagating in substantially opposite chirality;
a first transducer coupling a first port to the first and second
conductors such that an electromagnetic wave incident at the first
port excites first and second normal-mode partial wave fields on
the conductors;
a gyrotropic medium having a state of magnetization which is
variable between forward and reverse saturation levels,
sufficiently proximal to the conductors to introduce gyromagnetic
interaction with the wave fields, said gyromagnetic interaction
causing unequal phase change of the first and second partial wave
fields during propagation; and
a second transducer coupling the first and second conductors to a
second port, such that the unequal phases of the partial wave
fields arriving at the second transducer are compensated so as to
constructively interfere at the second port, the resultant
difference in phase change on transmission for a first wave
propagating from the first to the second port and a second wave
propagating from the second to the first port being substantially
an odd-integer multiple of 180 degrees.
28. A gyrator comprising:
a gyrotropic substrate having a state of magnetization which is
variable between forward and reverse saturation levels;
first and second substantially parallel conductors sufficiently
proximal to said substrate such that wave fields traversing the
conductors interact gyromagnetically with the substrate in a zone
of gyromagnetic interaction between said conductors;
a first transducer coupling a first port to first ends of said
first and second conductors such that an electromagnetic wave
incident at said first port excites first and second partial wave
fields propagating in substantially opposite chirality on said
conductors; said first and second partial wave fields undergoing
unequal phase changes during propagation through said zone; and
a second transducer coupling a second port to second ends of said
first and second conductors, said second transducer including a
balun structure such that said first and second partial wave fields
of unequal phases reinforce at said second port.
29. A gyrator comprising:
a gyrotropic substrate having a state of magnetization which is
variable between forward and reverse saturation levels;
first and second substantially parallel conductors sufficiently
proximal to said substrate such that wave fields traversing the
conductors interact gyromagnetically with the substrate in a zone
of gyromagnetic interaction between said conductors;
a first transducer coupling a first port to first ends of said
first and second conductors such that an electromagnetic wave
incident at said first port excites first and second partial wave
fields propagating in substantially opposite chirality on said
conductors; said first and second partial wave fields undergoing
unequal phase changes during propagation through said zone; and
a second transducer coupling a second port to second ends of said
first and second conductors, such that said first and second
partial wave fields of unequal phases reinforce at said second
port.
Description
BACKGROUND OF THE INVENTION
The term gyrator was introduced by Tellegen to designate the
concept of a circuit element embodying the essence of
nonreciprocity:
1. B. D. H. Tellegen: "The Gyrator, a New Electric Network
Element"; Philips Res. Rep. 3, 81-101 (1948).
Thus, where every reciprocal linear circuit device can be
represented by an appropriate combination of the four basic element
types, inductor, capacitor, resistor, and ideal transformer,
Tellegen envisioned that by augmenting these with a fifth element
type, the gyrator, every non-reciprocal linear device could be
represented as well.
The gyrator is a non-dissipative two-terminal device having forward
and reverse transfer phases which differ by 180.degree..
2. C. L. Hogan: "The Microwave Gyrator"; The Bell System Technical
Journal, Vol. XXXI, No. 1, 1-31 (January 1952).
This property might seem to violate the reciprocity principle, a
consequence of the symmetry properties with respect to time of the
fundamental laws of electromagnetism as expressed by Maxwell's
equations, which state that if a voltage is introduced at a first
location in a network, and a current is measured at a second
location, then the voltage/current ratio will be the same if the
locations of the voltage source and current sensor are
interchanged.
In fact, the gyrator represents no violation of this general
principle at all, but is instead a manifestation, under appropriate
conditions, of the distinctive constitutive properties of certain
media of electromagnetic propagation, called gyrotropic, which are
capable of undergoing a change in their influence on propagating
electromagnetic waves under reversal of their state of
magnetization. Magnetized ferrites and related magnetic oxides such
as YIG (yttrium-iron garnet), magnetoplumbites, and gaseous and
solid-state plasmas are examples of gyrotropic materials. Thus, the
conventional term "nonreciprocal" should not be taken literally,
but only as a convenient designation for this class of phenomena.
This property is exhibited with particular clarity in the
phenomenon of magnetic resonance induction:
3. F. Bloch, W. W. Hansen & M. Packard: "The Nuclear Induction
Experiment"; Phys. Rev. 70 474 (1946).
4. N. F. Ramsey, Nuclear Moments; Wiley, 1953.
and in microwave Faraday rotation of the polarization:
5. C. L. Hogan, "The Ferromagnetic Faraday Effect at Microwave
Frequencies and its Applications" Rev. Mod. Phys., Vol 25, pg 253
(1953).
6. B. Lax & K. J. Button, Microwave Ferrites and
Ferrimagnetics; McGraw-Hill, Sec. 12-1, p. 544 (1962).
The Bloch and Ramsey references [3,4 ]illustrate magnetic resonance
induction at radio frequencies. In these examples, a sphere or
other small specimen of gyrotropic material is placed at the center
of two mutually orthogonal concentric wire loops or coils mounted
in an electromagnet. When excited by an applied radio-frequency
signal in one of the coils, the magnetic moment of the specimen is
set into precessional motion, inducing an output signal in the
other coil. The sense of precession, clockwise or counterclockwise,
in response to an oscillating signal is related to the direction of
the static magnetizing field. If the static field is reversed, the
direction of magnetization and the sense of precession reverse.
This, in turn, reverses the phase of the electromagnetic coupling.
Likewise, if the roles of input and output connections are
interchanged, the direction of the magnetic field being left
unchanged, the phase relation between the incident and output
signal is reversed. This is the physical basis for the gyrator
action.
The principle is the same in the case of the Hogan and Lax &
Button references [5,6], which illustrate microwave Faraday
rotation. In these examples, a rod of gyrotropic material is
mounted on the axis of a circular-cylindrical waveguide and
magnetized axially. An incident linearly polarized microwave signal
undergoes rotation of its plane of polarization due to interaction
with the precessional magnetic motion which the signal induces in
the rod. Here again, the sense of polarization rotation is
determined by that of the precession which, in turn, depends on the
direction of magnetization. To demonstrate the performance of the
microwave gyrator, single-mode waveguides connected at the input
and output ends of the cylindrical guide are oriented about their
axes as polarizer and analyzer to accept polarizations at
90.degree. relative to one another. Similarly, conditions in the
rotator section, principally the diameter and composition of the
rod, and magnitude of the applied static magnetic field, are
arranged to produce a Faraday rotation angle of 90.degree.. Thus, a
signal incident from either end undergoes Faraday rotation so as to
be suitably polarized for transmission at the other end with only
incidental scattering, but the phase of the transmission is
opposite (differing by 180.degree.) in the case of the two
directions. Likewise, the phase changes for transmission in the two
directions are interchanged if the direction of magnetization is
reversed. These are the essential characteristics of the
gyrator.
The gyrator has long been a focal point of interest in relation to
microwave device and system technology, for its practical utility
as well as for its theoretical significance. Gyrators have served
as the basic nonreciprocal element in circulators and isolators,
which are indispensable in microwave systems of all kinds as means
to divide, combine and direct signals and to suppress unwanted
reflections in microwave systems. Actual physical embodiments of
the gyrator can perform as the non-reciprocal element in many
magnetic microwave devices; in addition, the gyrator also serves as
a powerful abstract concept for logical representation and analysis
in microwave circuit theory. Gyrators are also incorporated into
devices which provide other essential circuit functions, such as
magnetically controlled switching and phase shifting
operations.
Prior Art FIG. 1 illustrates a gyrator employed in a four-port
circulator which can function as a signal director, divider or
combiner or as an isolator or switch. The circuit is a bridge
configuration consisting of two "hybrid" or "magic" T junctions, T1
and T2, connected by two parallel lengths of transmission line 22A,
22B. Each junction T1, T2 includes an even symmetry port
(circulator ports 1 and 2) and an odd symmetry port (circulator
ports 3 and 4). Consider first an incident signal entering the
circulator via port 1. The incident signal 20 is divided into two
parts 20A, 20B which are conducted on two lines, or arms, of the
bridge 22A, 22B. In junction T2, the two signals 20A, 20B are
recombined into signal 24. A gyrator G is positioned in the first
arm 22A. The lengths of the two arms 22A, 22B are designed such
that, for an incident signal entering port 1, the even-symmetry
port of T1, and propagating in the direction from T1 to T2, the two
signals 20A, 20B remain in phase and are combined to emerge 24 at
the even-symmetry port of T2 (circulator port 2).
Consider now a second signal 26 incident on port 2 traversing
through the circuit in the opposite direction from junction T2 to
T1. Since the gyrator G furnishes a 180.degree. difference in phase
for this signal compared with the first direction of propagation,
it follows that, for a signal 26 incident at port 2 and divided
with equal phase into signals 26A, 26B, the two signals 26A, 26B
arrive at T1 precisely out of phase and are combined into signal 28
at the odd-symmetry port (circulator port 3) of T1 . This
illustrates the essential circulator action. Continuing the same
logic, a signal 30 incident at port 3 of T1 is divided with an
initial 180.degree. phase difference at junction T1 due to the odd
symmetry at that port and emerges at the odd-symmetry port of T2
(circulator port 4) as signal 32. Likewise, a signal 34 incident on
port 4 and divided with an initial 180.degree. phase difference at
T2 is affected by a further 180.degree. phase change due to the
gyrator G and emerges at the even-symmetry port of T1 (circulator
port 1) as signal 36.
The device described in the above example can be adapted for use as
an isolator by designating three ports of each T-junction for the
signal path, with the remaining port of each T-junction terminated
with matched attenuators. This circuit can also serve as a switch
or reversible circulator, by taking advantage of the
magnetic-control feature intrinsic to the gyrator: if the direction
of magnetization of the gyrotropic element is reversed, the gyrator
action is reversed and the 180.degree. phase difference between the
two arms of the bridge occurs for propagation in the opposite
direction, from T1 and T2, thereby reversing the circulation port
sequence outlined above, from 1-2-3-4-1 . . . to 1-4-3-2-1 . . .
.
Modern implementations of the gyrator generally require a
complicated structure, including a magnetic yoke external to the
microwave path, which makes them comparatively large in size and
weight and expensive to manufacture. For these reasons, modern
gyrators do not lend themselves well to the evolving technology of
microwave planar circuits, where minimization of size, weight, and
cost are essential.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for
forming a gyrator configured in planar circuit technology. The
apparatus of the invention comprises first and second parallel
transmission lines positioned proximal to a gyrotropic medium or
substrate. By virtue of their symmetry, the transmission lines are
adapted to propagate normal modes which are even and odd with
respect to the central plane of symmetry. Gyromagnetic interaction
leads to mixing or coupling of these modes, resulting in
elliptically-polarized normal modes of opposite chirality, where
chirality signifies right and left handedness. Excitation of a
combination of these modes results in Faraday rotation of the
polarization, which is most clearly evident in the zone of the
magnetized gyrotropic substrate between the two transmission lines.
The resultant effect is a conversion of the wave field from one of
even to odd symmetry in the course of transmission from input to
output.
Input and output transducers couple the ends of the transmission
lines to corresponding input and output ports. In an illustrative
embodiment, the input transducer transmits waves of even symmetry,
while reflecting waves of odd symmetry. The output transducer
behaves in an opposite manner. The widths and spacing of the
transmission lines can be selected by well-known methods of
guided-wave theory and practice to produce optimal impedance match
and device frequency bandwidth. Numerical analysis has indicated
that the invention exhibits inherent broadband capability.
In one aspect, the invention comprises first and second
substantially parallel conductors capable of supporting two normal
modes in the form of first and second elliptically polarized wave
fields propagating in substantially opposite chirality. A first
transducer couples a first port to the first and second conductors
such that an electromagnetic wave incident at the first port
excites waves on the first and second conductors with substantially
equal phase and amplitude (even mode). These waves can be regarded
as superposition of the first and second elliptically polarized
partial wave fields mentioned above. A gyrotropic medium is
positioned sufficiently proximal to the conductors to cause
gyromagnetic interaction between the magnetization in the medium
and the partial wave fields. The gyromagnetic interaction causes
unequal phase change of the first and second partial wave fields
during propagation. A second transducer couples the first and
second conductors to a second port such that the unequal phases of
the partial wave fields arriving at the second transducer are
compensated to substantially reinforce at the second port. In this
manner, the resultant difference in phase change for a first wave
propagating from the first to the second port and a second wave
propagating from the second to the first port is substantially an
odd-integer multiple of 180 degrees. In other words, the device
operates as a gyrator.
In another aspect, the invention comprises a device operating as a
gyrator including a gyrotropic substrate in proximity with first
and second substantially parallel conductors. The conductors are
capable of supporting first and second wave fields of substantially
opposite chirality. Each wave field travels on both conductors
simultaneously, as described above. The substrate is in sufficient
proximity with the conductors to interact gyromagnetically with the
wave fields. A first transducer is coupled to first ends of the
first and second conductors and a second transducer is coupled to
opposite second ends of the conductors. The first transducer is
preferably adapted to excite first and second partial wave fields
on the conductors, the partial wave fields being of substantially
equal phase, when a first electromagnetic wave is incident on the
first port. For propagation in the opposite direction, the first
transducer produces reinforcement at the first port of first and
second wave fields on the conductors of substantially equal phase.
The second transducer behaves in an opposite manner. The second
transducer is adapted to excite waves on the conductors of
substantially opposite phase, when a second electromagnetic wave
incident at the second port. For propagation in the opposite
direction, the second transducer produces reinforcement at the
second port of first and second wave fields on the conductors of
substantially opposite phase.
In a preferred embodiment, the input port is divided and
conductively connected to both of the two coupled transmission
lines in a symmetrical manner at the input transducer so as to
excite a wave of even symmetry. The output port, on the other hand,
is connected to the transmission lines in an unsymmetrical manner
at the output transducer so as to provide the function of a
"balun", or balanced-to-unbalanced transducer, which shifts waves
of opposite phase on the two coupled lines into equal phase. Thus,
they will reinforce, not cancel, when they flow together at the
output port. For example, at the output port, one of the
transmission lines is connected directly to the output and the
other is connected through a half-wavelength "hairpin" extension,
bringing the out-of-phase components from the transmission lines
into equal phase so as to reinforce, or otherwise interfere
constructively at the output. The concept of a balun is well known
in transmission-line art and may be realized in various ways for
planar applications. Note that the terms "input" and "output" as
used herein when referring to ports and transducers are used for
the purpose of clarity only and are freely interchangeable.
In a second preferred embodiment, the magnetization is confined
within the gyromagnetic structure, such that the structure is
magnetized in its plane, parallel to the orientation of the
transmission lines. This enhances the design and performance of the
planar circuit, lending this embodiment well to the emerging
interest in high temperature superconductors. The present invention
is operable in a partial or fully magnetized state. Where the
gyrotropic medium is formed in a closed path, it can be magnetized
by an initial latching current and operated in a remanent state,
that is, without an external magnet or coil.
The invention has application as a magnetically-controlled phaser
or switch, or as a component in a circulator or isolator
implemented in planar microwave technology. The invention is
especially attractive to application in miniaturized planar
microwave devices, for example MMICs.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the
invention will be apparent from the more particular description of
preferred embodiments of the invention, as illustrated in the
accompanying drawings in which like reference characters refer to
the same parts throughout the different views. The drawings are not
necessarily to scale, emphasis instead being placed upon
illustrating the principles of the invention.
FIG. 1 is a schemtic illustration of a prior-art four-port
circulator employing a gyrator.
FIG. 2 is a perspective view of an illustrative embodiment of a
planar gyrator in accordance with the present invention.
FIG. 3 is a perspective view of a gyrator implemented on a
substrate in the form of a closed magnetic circuit magnetized
within its plane in accordance with the present invention.
FIGS. 4A and 4B are perspective sectional views of the electric and
magnetic field configurations of even and odd modes of propagation
respectively.
FIGS. 5A, 5B, 5C, and 5D illustrate a planar gyrator having a
single layer of gyrotropic material, dual layers of gyrotropic
material, dual layers each with ground planes, and with conductors
embedded in the gyrotropic material respectively in accordance with
the present invention.
FIG. 6A and 6B are perspective illustrations of alternative balun
embodiments in accordance with the present invention.
FIG. 7 is a chart of computed transmission and reflection
amplitudes of a simplified computational model of the planar
gyrator with a designed band center at 10 GHz in accordance with
the present invention.
FIG. 8A and 8B are charts of the transmission and reflection
coefficients of the planar gyrator on the plane of complex numbers
for forward and reverse magnetizations illustrating gyrator action
in accordance with the present invention.
FIGS. 9A and 9B are charts of the transmission and reflection
phases as functions of frequency for forward and reverse
magnetizations respectively, illustrating classic gyrator action in
accordance with the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
With reference to FIG. 2, the gyrator 50 hereafter described in
detail is referred to as a planar-circuit gyrator, or planar
gyrator. In a preferred embodiment, the planar gyrator 50 comprises
a coupled pair of substantially parallel transmission lines 64A,
64B of preferably equal width in sufficient proximity with a
gyrotropic medium 52 so as to interact gyromagnetically therewith.
The transmission lines 64A, 64B are coupled to a first port 60 at a
first end and a second port 72 at a second end by first and second
transducers 62, 70 respectively.
The gyrotropic medium 52 preferably comprises a ferrite substrate
magnetized in its plane, that is, having a magnetization M parallel
to the direction of the transmission lines 64A, 64B as shown in
FIGS. 2 and 3. The transmission lines 64A, 64B comprise two
parallel conducting strips of equal width, for example microstrip
or balanced stripline, deposited on a first surface 53 of the
substrate 52. An opposite surface 55 of the substrate 52 is coated
with a ground plane 54. The strips 64A, 64B are preferably
chamfered 63 at the corners to reduce unwanted or spurious
reflections.
The coupling length L between the transmission lines 64A, 64B is
preferably approximately equal to one-half wavelength, in terms of
the average of the even- and odd-mode propagation constants, at the
center of the frequency range contemplated for the design. The
widths and spacing of the coupled strips can be selected to yield
favorable performance in terms of match and bandwidth.
The conceptual resemblance between this arrangement and the
waveguide Faraday rotator cited above may be seen by considering
the polarization of the field in the magnetic medium in the
vicinity of the gap 66 between the transmission lines 64A, 64B.
Referring to FIG. 4, note that in the case of the even mode, FIG.
4A, in the zone 65 between and beneath the two transmission lines
64A, 64B, with electric fields 63A, 63B oriented as shown, the
resultant microwave magnetic field 67A is predominantly directed
horizontally; in that same region, in the case of the odd mode,
FIG. 4B, it is predominantly vertical 67B. Superposition of the two
modes with equal phase would result in a diagonal direction of
polarization in the zone 65; but when the two are superposed with a
90 degree relative shift in phase, as occurs as a result of
gyromagnetic interaction, the result is an elliptical polarization
with the direction of the field at a given location rotating as the
wave propagates. In that zone 65, the microwave field is comparable
to that near the center of the circularly cylindrical waveguide as
contemplated in the case of waveguide Faraday rotation devices.
The gyromagnetic interaction is a stimulation by the propagating
microwave magnetic field of the atomic magnetic moments which are
responsible for the magnetic properties of the substrate material.
The response is a gyroscope-like precessional motion of the moments
with a clockwise, or right-handed, sense (chirality) relative to
the direction of magnetization of the substrate. This right-handed
sense is dictated by a fundamental relation between the intrinsic
angular momentum and magnetic moment of the atomic electrons. A
wave which is circularly polarized in the sense synchronous with
the precessional motion interacts strongly with the medium and
normally undergoes retardation of its velocity of propagation,
while a wave circularly polarized in opposition to the precession
interacts only weakly, and its velocity is normally affected to a
lesser degree. The phenomenon is most striking under conditions of
magnetic resonance, but those conditions are not necessarily the
most favorable for device performance.
A preferred magnetic state of the substrate is that based on the
property of hysteresis, whereby the medium remains magnetized after
an internal magnetic field furnished by means of an externally
imposed solenoid or electromagnet has been applied and then
removed. This condition is represented by the direction and
magnitude of the magnetization, or magnetic moment density M of the
gyrotropic medium. The value of this parameter required for
successful gyrator function is customarily expressed by the ratio
.kappa./.mu., where .mu. and .kappa. denote respectively the
diagonal and off-diagonal components of the gyromagnetic
permeability tensor customarily known as the Polder tensor:
7. D. Polder, "On the Theory of Ferromagnetic Resonance", Phil Mag,
Vol. 40, pg. 99 (1949)
Under preferred conditions of operation, with the medium in its
remanent state and with a static internal magnetic field which is
very small or altogether absent, this ratio is approximately equal
to f.sub.M /f, where f.sub.M is proportional to M [f.sub.M
=.gamma.(4.pi.M), where .gamma. is the gyromagnetic constant] and f
is the frequency of the microwave signal.
The preferred direction of magnetization is within the plane of the
planar-circuit substrate and aligned parallel to the direction of
propagation of the coupled transmission lines. The magnitude of
f.sub.M /f must be great enough to produce full transmission
through the device; i.e., full conversion of the incoming wave of
even symmetry into the outgoing wave of odd symmetry, with minimum
reflection of the incident wave. The magnitude of f.sub.M /f must
be less than unity, f.sub.M /f<1, however, in order to avoid the
onset of a magnetic-resonance-related energy dissipation effect,
known as low-field loss, to which magnetic media are susceptible
when less than fully magnetized.
The selection of material of suitable chemical and crystallographic
composition in order to meet the above requirements can be
accomplished by methods and principles which are well known to
those versed in magnetic microwave technology.
Referring back to FIG. 2, the transmission lines 64A, 64B are
terminated at each end by first and second transducers 62, 70 which
couple the transmission lines 64A, 64B to first and second ports
60, 72. In a preferred embodiment, the first transducer 62
transmits the even mode of a signal 82 within the coupled line
region propagating toward the first transducer 62 with
substantially no dissipative loss or reflection, while blocking the
odd mode with substantially complete reflection and negligible
dissipation. The second transducer 70 at the opposite end behaves
in an opposite manner, transmitting the odd mode and blocking the
even mode of signal 81. Similarly, a first electromagnetic input
wave 80 arriving at the first port 60 excites first and second
normal mode partial wave fields on the conductors 64A, 64B with
substantially equal phase and amplitude, while a second
electromagnetic input wave 86 arriving at the second port 72
excites the partial wave fields with substantially opposite
phase.
The principle of the planar gyrator 50 does not depend on ideal
fulfillment of these scattering requirements at the transducers.
The existence of a sufficient distinction between the even- and
odd-mode scattering amplitudes will suffice. Device performance is
related to the degree of this distinction. Analysis of the planar
gyrator performance takes into consideration the scattering of the
incident wave and of counterpropagating internal waves 81, 82 at
the transducers 62, 70 together with propagation of the normal
modes along the length of the coupled transmission lines 64A,
64B.
A preferred embodiment of the planar gyrator includes a planar
circuit as described above, with the magnetized substrate 52
comprising one leg of a closed planar magnetic circuit 79 capable
of remaining permanently magnetized or "latched" in its remanent
state as shown in FIG. 3. An optional current winding 78 serves to
reverse the sense of magnetization, if such switching capability is
called for in the application.
FIGS. 5A-5D illustrate cross-sectional views of alternative planar
technologies. FIG. 5A illustrates a planar gyrator having a single
gyrotropic substrate 52, coupled conducting transmission line
strips 64A, 64B, and a ground plane 54.
In FIG. 5B, a second gyrotropic layer 52B, magnetized in a
direction opposite the first layer 52A, is applied to upper surface
of the circuit 64A, 64B. Such a configuration confers several
significant advantages. First, it mitigates the disadvantageous
effects of an inhomogeneous dielectric cross-section giving rise to
unequal propagation constants for the even and odd modes, which
tends to degrade the gyrator performance. Second, both the upper
52B and lower 52A gyrotropic layers contribute to the nonreciprocal
gyrator action, increasing the gyrotropic effect by at least a
factor of two. Third, in this configuration the layers could be
arranged to form the forward and return legs of the same magnetic
circuit, leading to a very efficient high remanent state with
low-energy and high-speed switching. If this dual gyrotropic layer
arrangement is incompatible with the magnetic circuit requirements
or other constraints of the application in question, a dielectric
overlay applied to the upper surface having a dielectric constant
similar to that of the ferrite substrate would still confer the
first advantage mentioned above.
The embodiment of FIG. 5C adds a second ground layer 54B to the
upper layer of gyrotropic material 52B. The resulting balanced
stripline configuration confers additional confinement and
shielding of the device and would be expected to lead to optimum
strength of the gyromagnetic interaction. In FIG. 5D, the
conductors 64A, 64B are embedded in the gyrotropic material 52.
Design considerations of the first and second dual-mode transducers
62, 70 will now be described in further detail. Selective coupling
of the incoming signal 80 to the even mode is relatively easily
accomplished, for example, by a symmetrical division of the input
port 60 to form the coupled transmission line pair 64A, 64B as
shown in FIG. 2. Such a simple configuration intrinsically
represents a short circuit to the odd mode (since it joins together
points of positive and negative polarity of that mode) while
lending the capability of providing a favorable match for
excitation of the even mode. Local fringing reactance giving rise
to a minor mismatch can be compensated by capacitive or inductive
steps as generally practiced in the well-known filter and coupler
arts and as described in the following references, incorporated
herein by reference:
8. Brian C. Wadell, Transmission-Line Design Handbook; Artech
House, 1991.
9. K. C. Gupta, R. Garg & I. J. Bahl, Microstrip Lines and
Slotlines; Artech House, 1979.
10. G. Matthaei, L. Young & E. M. T. Jones, Microwave Filters
Impedance-Matching Networks, and Coupling Structures; McGraw-Hill,
1964. Reprint: Artech House, 1980.
Design of the odd-mode transducer 70 at the output end presents
additional challenges. A short circuit for the even mode at that
end can be created, for example, by inserting a grounded vertical
pin 71 or vane symmetrically disposed between the two transmission
lines 64A, 64B in the vicinity of the odd-mode transducer. An
unsymmetrical element constituting a "balun" brings the component
signals of opposite polarity from the two coupled lines into equal
phase at the output port 72. This has the effect of coupling the
odd mode to the output port 72, and, with appropriate matching
features as generally practiced, prevents or minimizes reflection
of the odd mode back into the transmission lines 64A, 64B. In the
examples shown in FIGS. 2 and 3, the balun takes the form of a
"hairpin" 68 of total length one-half wavelength inserted into one
of the lines 64B. In addition to the hairpin example, two
alternative embodiments of the balun principle are shown in FIGS.
6A and 6B. In FIG. 6A, a strip structure 92, electrically analogous
to an E-plane port in rectangular-tube waveguide, couples waves of
the odd mode on the two coupled strips 64A, 64B into a
single-conductor planar line 72. A central vane 90 or pin 71 (see
FIG. 2) provides a short-circuit reflector for the even mode. In
FIG. 6B, structures 94, 96 constituting "lumped elements", a
capacitor 94 on conductor 64A and an inductor 96 on conductor 64B,
provide phase discontinuities of approximately -90 degrees and +90
degrees, respectively, at the output port 72, adding to give the
total of approximately 180 degrees required in order to produce
substantial constructive interference at the output line 72 for the
odd mode and substantial destructive interference for the even
mode. Each of these, as well as other possible designs, presents
its own tradeoffs between advantages and disadvantages whose
suitability depends on the specific application in question.
Results of a computational analysis are illustrated in FIGS. 7-9.
FIG. 7 is a chart of transmission and reflection amplitudes as
functions of frequency for a computational model of the planar
gyrator having a band center near 10 GHz, assuming a homogeneous
medium. Illustrative points 105, 106 at a frequency of 8 GHz are
indicated for comparison in FIGS. 7-9. Sharp nulls 102 are apparent
in transmission at frequencies at which the length L of the coupled
transmission line section is approximately equal to an odd-integer
multiple of a quarter-wavelength. At frequencies between these
features, i.e., over ranges centered at integer multiples of a
half-wavelength, there is generally a broad region 104 of
relatively low reflection, at least part of which may present an
excellent match. These matching regions are the favorable operating
ranges of the device. Except at the narrow nulls 102 mentioned, the
device exhibits classic gyrator performance; namely, 180.degree.
phase differential between the two directions of propagation and
180.degree. reversal of the transmission phase in either direction
upon reversal of the direction of magnetization.
FIG. 8A is a chart of the phase and amplitude of the transmission
coefficient A1 . . . A2 and reflection coefficient B1 . . . B2 over
an operating range of 5 to 16 GHz, represented on the plane of
complex numbers. FIG. 8A refers to the case of magnetization of the
substrate in the positive direction. Thus, for example, the radial
line representing a transmission phase angle of substantially -120
degrees intersects curve A at a point 105 whose radial position
indicates a transmission coefficient amplitude of 0.98; and the
radial line representing a phase angle of substantially +60 degrees
intersects curve B at a point 106 indicating a reflection
coefficient amplitude of 0.33. The corresponding points in FIG. 7
show that these values occur at a frequency of 8.0 GHz.
In FIG. 8B, the transmission coefficient A1 . . . A2 and reflection
coefficient B1 . . . B2 are illustrated with magnetization reversed
from that of FIG. 8A. The phase of the reflection coefficient B1 .
. . B2 remains unchanged, as expected. On the other hand, curve A1
. . . A2 is turned over such as to indicate that the phase of the
transmission coefficient is shifted by 180 degrees at every
frequency, illustrating classic gyrator action. This same result is
obtained if the magnetization is left unchanged and identities of
the input and output ports are interchanged.
FIGS. 9A and 9B are charts of the phase angle as in FIGS. 8A and 8B
respectively, plotted versus frequency. The point 105 in FIG. 9A at
8.0 GHz, indicating a transmission phase value of -120 degrees,
corresponds to the point 105 in FIG. 8A cited above. In FIG. 9B,
the magnetization is reversed and, comparing FIGS. 8A and 8B, 180
degree transmission phase change between positive and negative
magnetizations is evident, illustrating gyrator action. The
illustrative point 105 at 8.0 GHz has changed from a phase of -120
degrees to a phase of 60 degrees.
Within the above general sketch of the concept of a planar gyrator,
considerable flexibility exists for optimization and adaptation to
specific frequency bands, geometrical constraints and system
objectives by means well known to those skilled in the art.
The present invention is further applicable for use as a
magnetically-controlled switch, since in the unmagnetized state the
input port and output port may be made highly uncoupled over a
broad band. For the "off" state of the switch, the substrate is
demagnetized. In the absence of gyromagnetic coupling, the entering
even mode undergoes no conversion to an admixture with the odd
mode, and is therefore totally reflected at the output transducer.
When the medium is switched to an appropriate state of
magnetization, transmission occurs over a substantially broad band,
as has been shown above in FIG. 7 and the associated
description.
Although the use of circular polarization in planar coupled lines
was understood in the past with regard to the meanderline phase
shifter for example:
11. Fred J. Rosenbaum, "Integrated Ferrimagnetic Devices", Advances
in Microwaves, Vol. 8, pp 203-294, Academic Press, 1974;
12. G. T. Roome & H. A. Hair, "Thin Ferrite Devices for
Microwave Integrated Circuits", IEEE Trans. MTT, Vol 16, pp 411-420
(1968);
the present invention recognizes that a gyrator can be formed by
means of a different configuration; namely parallel planar coupled
lines in combination with even and odd mode transducers at the ends
of the lines.
There is a tendency for current to be concentrated at the sharp
edges of a conductor, leading to undesirable ohmic conductive
energy loss. This phenomenon is a problem in a typical
photolithographically deposited planar-circuit strip which
generally has not only more or less sharp, but furthermore ragged
or uneven edges resulting from the etching process. One technique
for avoiding this problem is to employ high- or low-temperature
superconducting technology, as described in U.S. Pat. No.
5,484,765, incorporated herein by reference. In another technique,
the strip conductors are formed to be generally elliptical in
cross-section so as to create a smooth, rounded profile, and placed
on or embedded in the substrate. The rounded corners of the
conductor result in reduced current concentration and thereby
reduced loss. The use of gold or other conventional (i.e.,
non-superconducting) rounded-profile conductors in combination with
cryogenic temperatures is still another effective means for
reducing conduction loss in planar circuit devices. Note that for
purposes of the present disclosure, the term "planar", when
referring to conductors, includes and is not limited to the
following conductors: standard photolithographically deposited
planar conductors; planar conductors of elliptical cross-section;
and planar superconductors.
While this invention has been particularly shown and described with
references to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the invention as defined by the appended claims.
* * * * *